Since the advent of electronic instruments and the cathode ray oscilloscope, electrical signal (voltage or current) monitoring has been by watching an actual "trace" of the signal on a cathode ray tube or `scope. However, as signal frequencies increased and as such signals became more complex in waveform, such equipment became less equal to the task of good waveform analysis.
Waveform "sampling" followed. Waveform analysis by sampling, which per se is performed digitally by known devices, involves picking off discrete or "sample" points of a waveform, storing those points in memory and recalling them for later analysis.
Equipment of the general type is depicted in U.S. Pat. No. 4,142,146 (Schumann et al.); U.S. Pat. No. 4,625,283 (Hurley) and U.S. Pat. No. 5,222,028 (Labarre et al.). The system and method of the LaBarre et al. patent identifies a waveform trailing edge by comparing waveform data to predetermined slope criteria. Once a trailing edge (and, thus, the end of a pulse) is identified, the immediately preceding leading edge is assumed to define the beginning of the pulse and, together with the trailing edge, defines pulse height.
Pulse width is calculated as the time between the leading edge 50% point and the trailing edge 50% point. The rise time is calculated to be the time required for the signal to rise from 10% to 90% of its normal pulse amplitude.
The Hurley patent involves an apparatus and method for measuring alternating current. Sampling of a waveform occurs as such waveform crosses negative and positive reference voltage values. The Hurley apparatus and method correlate amplitude values of the waveform with measured time intervals between crossings. The Hurley patent has what may be an ambiguity or outright error in its reference to an alternating current which crosses reference voltage values.
The Schumann et al. patent involves an apparatus which is particularly useful when the signal being analyzed has a frequency more than half the frequency of sampling. The Schumann et al. apparatus samples the waveform at irregular intervals.
While the apparatus and methods depicted in the aforementioned patents have been generally suitable for their intended purpose, they tend to be characterized by certain disadvantages. For example, the method of the LaBarre et al. patent makes certain assumptions about where a particular waveform "pulse" begins rather than taking actual "point" measurements to positively determine such beginning. And such patent also determines pulse width by techniques which may be less than precise.
Laying aside the matter of possible ambiguity or error, the method described in the Hurley patent samples a waveform as it crosses negative and positive reference voltage values. Often, there are situations where sampling is desired at other instants.
When considered in the framework of an overall system for testing printed circuit boards, e.g., a system which includes an environmental test chamber, the known prior art apparatus appears not well suited to be integrated with such system. And the known prior art apparatus and methods seem to lack flexibility in enabling determination of a wide variety of waveform parameters, e.g., rise time, fall time, frequency, period, maximum and minimum waveform values and others.